Fungal lignin peroxidase <p>Peroxidases are haem-containing enzymes that use hydrogen peroxide asthe electron acceptor to catalyse a number of oxidative reactions. Peroxidases are found in bacteria, fungi, plants and animals. Fungal ligninases, or lignin peroxidases (LiPs), and manganese-dependent peroxidases (MnPs), are extracellular haem enzymes involved in the degradation of lignin. In MnP, Mn²⁺ serves as the reducing substrate [<cite idref="PUB00001743"/>].</p><p>It is commonly thought that the plant polymer lignin is the second most abundant organic compound on Earth, exceeded only by cellulose. Higher plants synthesise vast quantities of insoluble macromolecules, including lignins. Lignin is an amorphous three-dimensional aromatic biopolymer composed of oxyphenylpropane units. Biodegradation of lignins is slow - it is probable that their decomposition is the rate-limiting step in the biospheric carbon-oxygen cycle, which is mediated almost entirely by the catabolic activities of microorganisms. The white-rot fungi are able extensively to decompose all the important structural components of wood, including both cellulose and lignin. Under the proper environmental conditions, white-rot fungi completely degrade all structural components of lignin, with ultimate formation of CO₂ and H₂O. The first step in lignin degradation is depolymerisation, catalysed by the LiPs (ligninases). LiPs are secreted, along with hydrogen peroxide (H₂O₂), by white-rot fungi under conditions of nutrient limitation. The enzymes are not only important in lignin biodegradation, but are also potentially valuable in chemical waste disposal because of their ability to degrade environmental pollutants [<cite idref="PUB00001743"/>].</p><p>To date, 3D structures have been determined for LiP [<cite idref="PUB00002822"/>] and MnP [<cite idref="PUB00002886"/>] from <taxon tax_id="5306">Phanerochaete chrysosporium</taxon> (White-rot fungus), and for the fungal peroxidase from <taxon tax_id="5451">Arthromyces ramosus</taxon> [<cite idref="PUB00003315"/>]. All these proteins share the same architecture and consist of 2 all-alpha domains, between which is embedded the haem group. The helical topography of LiPs is nearly identical to that of yeast cytochrome c peroxidase (CCP) [<cite idref="PUB00005246"/>], despite the former having 4 disulphide bonds, which are absent in CCP (MnP has an additional disulphide bond at the C terminus).</p>